One of the most terrestrial electric power-generating solar cells currently are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing to form a metal paste. An example of this method of production is described in the following with FIG. 1. The present invention is adaptable not only to the example shown as FIG. 1 but also to the n-base type or back-contact type solar cell.
FIG. 1A shows a p-type silicon substrate, 10.
In FIG. 1B, an n-type diffusion layer, 20, of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the phosphorus diffusion source. In the absence of any particular modification, the diffusion layer, 20, is formed over the entire surface of the silicon substrate, 10. This diffusion layer typically has a sheet resistivity on the order of several tens of ohms per square, and a thickness of about 0.3 to 0.5 μm.
After protecting one surface of this diffusion layer with a resist or the like, as shown in FIG. 1C, the diffusion layer, 20, is removed from most surfaces by etching so that it remains only on one main surface. The resist is then removed using an organic solvent or the like.
Next, a silicon nitride film, 30, is formed as an anti-reflection coating on the n-type diffusion layer, 20, to a thickness of typically about 700 to 900 Å in the manner shown in FIG. 1D by a process such as plasma chemical vapor deposition (CVD).
As shown in FIG. 1E, a silver paste, 50, for the front electrode is screen printed then dried over the silicon nitride film, 30. In addition, a backside silver or silver/aluminum paste, 70, and an aluminum paste, 60, are then screen printed and successively dried on the backside of the substrate. Firing is then carried out in a furnace at a temperature of approximately less than 1000° C. for several seconds or for several minutes.
Consequently, as shown in FIG. 1F, aluminum diffuses from the aluminum paste into the silicon substrate, 10, as a dopant during firing, forming a p+ layer, 40, containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
The aluminum paste is transformed by firing from a dried state, 60, to an aluminum back electrode, 61. The backside silver or silver/aluminum paste, 70, is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer, 40. Because soldering to an aluminum electrode is impossible, a silver back electrode is formed over portions of the back side as an electrode for interconnecting solar cells by means of copper ribbon or the like. In addition, the front electrode-forming silver paste, 50, sinters and penetrates through the silicon nitride film, 30, during firing, and is thereby able to electrically contact the n-type layer, 20. This type of process is generally called “fire through”. This fired through state is apparent in layer 51 of FIG. 1F.
Glass frit is added to the conductive paste used in silicon solar cells such as the above to obtain sufficient adhesive strength on the substrate even when the firing time is short, and conductive pastes in which conductive metallic powders such as silver, glass frit (glass frit), and various additives are dispersed in an organic vehicle are usually used. Further, an insulating protective film is sometimes formed on the surface of semiconductor substrates in solar cells, and a conductive paste in which glass frit is added is used also as the conductive paste used on the protective film, so that the insulating protective film is dissolved away during firing. However, there is a problem in that such glass frits often soften and flow during the firing process and become segregated in the interface between the substrate and electrode, resulting in the formation of an insulating layer, which increases electrode resistance. To avoid this problem, the amount of glass frit that is added may be lowered or the firing temperature may be lowered, but these options may lead to reduced adhesive strength between the electrode and conductor substrate or between the electrode and solder. The insulating protective film could be insufficiently dissolved away during firing. There is a trade off between decreasing the electrical resistance and increasing the adhesive strength, and a resulting conventional problem is a difficulty of multaneously bringing about low resistance and high adhesive strength. Recently, shallow-emitter type solar cells with shallow layers on the light-receiving side have been attracting attention, with the promise of superior wafer properties, but as ensuring both the conductivity and adhesion of electrodes is even more difficult in this type, there is a need for electrodes endowed with both lower resistance and higher adhesive strength. The following means have been proposed for solving these problems relating to electrodes for solar cells.
JP2001-313400; discloses how to solve adhesion and resistance problem in a solar cell electrode. The electrode material includes one or more powders from among titanium, bismuth, cobalt, zinc, zirconium, iron, and chromium or its oxides.
JP2001-127317 discloses conductive paste containing two kinds of glass frit which have low softing point glass frit and high softing point glass frit in order to obtain an electrode with strong adhesion.
JP2008-109016 discloses glass composision in a conductive paste which gives strong adhesion between Si wafer and an electrode. The glass composition comprises 5 wt % to 10 wt % of ZnO, 70 wt % to 84 wt % of Bi2O3, more than 6 wt % of B2O3 plus SiO2.
JP2008-042095 discloses a multilayer structure of electrode to enable sufficient adhesion between the Si wafer, the electrode and solder. The lower layer attaching to the Si wafer contains one or more than two of Ti, Bi, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Si, Al, Ge, Sn, Pb, Zn as an additive.
JPH11-329070 discloses conductive paste which is capable of reliably forming an electrode having a low contact resistance and strong adhesion. The conductive paste contains crystalline composite oxide. There is a need for conductive pastes with low resistance and high adhesion to electrode surfaces.